We received a question from a reader who would like to know how much time a worm can spend underwater before drowning. The short answer is that, so long as the water has a high enough concentration of oxygen, most worms can live quite happily under water. When the soil that an earthworm is living in becomes very wet, it may choose to come to the surface to move around. It’s a lot easier to move on the surface of the ground than it is to slog through mud.
The term “worm” is rather vague, but we will assume that she is referring to the common earthworm. One of the many benefits to being an earthworm is that earthworms cannot drown (because they have no lungs). Instead of breathing, earthworms actually absorb oxygen through their skin via diffusion. So, if the earthworm’s skin is moist and the air or water around it contains a higher concentration of oxygen than its blood, then there is no problem. In fact, they have the opposite problem. It is not uncommon to find an earthworm that has suffocated because its skin became too dry and it was unable to absorb oxygen.
Then, there are aquatic worms (such as those in the class Oligochaeta) such as those that you’ll find in a fish tank. These live their entire lives underwater.
In the end, the answer to our reader’s question is going to depend on the type of worm she’s talking about, but the short answer is that even terrestrial worms can live for quite a long time underwater, and there are other species that spend their entire lives underwater. So, if she’s concerned about the worms in her garden having a tough time when it rains, she can rest easy knowing that they’re going to be just fine. To humans, rain makes transportation more difficult. But, if you’re an earthworm rain just means that you can travel above the ground instead of through it!
In general, earthworms are in much more danger when in a dry environment. So, if she happens to see any of the little fellows stranded on a dry sidewalk then it would be kind to move them back to the soil.